Seasonal variations in serum vitamin D according to age and sex

Background: Exposure to sunlight is the main source of vitamin D production. This study was performed to assess the status of serum vitamin D across the different seasons in geographic region of Babol, with latitude of 36 in northern Iran.

Methods: The study - female participants were 15 years old and above selected prospectively according to the inclusion criteria who attended an Outpatient clinic. The serum 25-hydroxyvitamin D 25-OHD was measured with enzyme-linked immunosorbent assay (ELISA). Serum OHD <20 ng/ml was considered as vitamin D deficiency. Serum 25-OHD levels were compared across various seasons according to sex and age. Proportions of serum 25-OHD deficiency were also compared between the various seasons according to sex and age.

Results: A total of 576 females and 120 males with respective mean age of 44.8±14.1 and 47.8±29 years entered into the study. The mean serum 25-OHD was 20.8±27 ng/ ml, the prevalence of serum 25-OHD deficiency was 70.1%. In women compared with men, the mean serum 25-OHD was significantly lower but the proportion of deficiency was not significant (20.6±24.9 ng/ml vs 23.2±31.4 ng/ l. p=0.021 and 70.8% vs 67.5% p=0.60 respectively). The mean 25-OHD nd proportion of deficiency did not vary across the different seasons with regard to age. However, in the summer and in the autumn, the women had significantly lower serum 25-OHD concentrations than the men (p= 0.021and 0.016 respectively).

Conclusion: The findings of this study indicated no significant seasonal variations of vitamin D in geographic regions of Babol. However, during the autumn and the winter months, the women are at high risk of vitamin D deficiency which corresponds to nadir of serum 25-OHD levels.